Hibernation of computer systems generally refers to powering down a computer in a special way that preserves the state of the computer's configuration. Hibernation works by saving the volatile memory contents to a file on the computer's hard disk drive before shutting down. When power is restored, the file is read back into the memory and device states and services are restored, whereby the computer resumes the state it was in just before the hibernation, i.e., as if power was never turned off. Hibernation is especially popular for powering off mobile computers (e.g., notebooks and laptops) and other computers that are frequently turned on and off, because, unlike shutdown, hibernation automatically preserves opened documents, desktop settings, and other resources that have not been otherwise saved. In addition, a resume from hibernated state is often much faster than a full system startup. Hibernate is also preferable in many instances over a suspend mode that preserves the memory's contents by providing sufficient power to the RAM, because in suspend mode the RAM contents are lost if power is ever interrupted.
While hibernation is thus a valued feature, it is relatively slow because of the large amounts of data (e.g., in contemporary systems the memory is typically at least sixty-four megabytes) that need to be transferred to and from the disk. Hibernation takes on the order of twenty to forty seconds for entering hibernation and ten to fifteen seconds to resume from hibernation, depending on the model of the hard disk and the amount of installed memory. Many users are unsatisfied with the amount of time the hibernation process takes, and indeed, virtually every user of this feature would prefer that it was a faster process.